Bush was beginning the first leg of a five-country Latin American visit designed to bolster U.S. standing in the region and counter the growing influence of Venezuelan President Hugo Chavez.

The trip is the president's longest to date in Latin America, a region many Bush critics say has been largely ignored as the White House focused on Iraq and the Middle East.

In Brazil, Latin America's largest and most populous nation, Bush and Brazilian President Luiz Inacio Lula da Silva are expected to unveil a new commercial partnership centering on ethanol, a plant-based gasoline substitute that Bush is championing as a future alternative to fossil fuels.

But in their pre-arrival protest, many here yesterday hoisted banners likening Bush to Hitler and warning Bush to keep his "hands off" Venezuela, while also criticizing any potential U.S.-Brazilian biofuels pact as a plot by Washington to grab Brazilian resources.

"For Bush, this is a matter of getting cheaper fuel and getting out of the mess in the Middle East," said Cristiana Coimbra, 35, a professional translator who wore a sticker featuring Bush with a swastika.

Hundreds of riot police flanked the protesters, and the scent of tear gas hovered along the march route. Several injuries were reported, but there was no immediate word on the condition of those injured.

Brazilian police were also deployed in force to clear the route that Bush was scheduled to take from the airport to his hotel near downtown. Reports here indicated that as many as 4,000 law enforcement officers were participating in one of the largest security operations in recent memory.

On his last visit to the continent, in 2005, Bush never witnessed the huge demonstrations criticizing his presence at a hemispheric economic summit in Argentina.

Anti-Bush rallies seem likely to follow Bush on his Latin America tour. Bush and first lady Laura Bush have planned a busy agenda for today, meeting with the Brazilian president and with a variety of community representatives.

Da Silva, standard-bearer of the leftist Workers Party, has veered to the center since first being elected in 2002 and is regarded as a firm U.S. ally committed to orthodox economic policies.

The two presidents are scheduled to deliver a joint statement on biofuels technology at a facility of Petrobras, the state-run energy firm.